Metal components of structures are susceptible to defects, such as due to imperfect manufacture, corrosion, fatigue, wear, damage, etc. To prevent catastrophic failure of such structures, metal components may be visually inspected to identify defects before a failure occurs. However, many structures are not easily inspected due to being buried underground or beneath the sea, or due to being embedded within other materials such as concrete. For large infrastructure that contains metal components, visual inspection may be impractical or too costly to perform routinely.
Many ferromagnetic objects, including steel pipe, act as weak permanent magnets even when not intentionally magnetized; for example, magnetic dipoles in steel may partially orient to the Earth's magnetic field after cooling below the Curie temperature when cast or hot-rolled in the foundry. Magnetic fields present in ferromagnetic objects as stray byproducts of their manufacture are known herein as parasitic fields. The Earth's magnetic field also induces magnetic fields in ferromagnetic objects. These magnetic fields permit detection of ferromagnetic objects from a distance. Magnetic exploders for naval mines and torpedoes have been designed to detect magnetic fields from large ferrous objects, such as warships, since 1917, although both German and American magnetic exploders were problematic when used in combat on torpedoes in 1939-1943. Magnetic exploders, however, are merely intended to detect the object from a distance, not to detect or analyze defects in that object.
Magnetic particle inspection is well known as a method for detecting cracks in objects. In this technique, a ferromagnetic object is placed in a magnetic field, and magnetic particles, such as iron filings, are applied to the object. The magnetic field may be provided by passing an electric current through the object, or by placing the object in a field provided by an electromagnet. If a crack is present, the magnetic particles cluster near the crack. Field strengths used for magnetic particle inspection are typically much greater than the Earth's magnetic field, or those parasitic fields that may be present in ferromagnetic materials.